Why is plant diversity important?
I can come up with lots of reasons, including the value to pollinators, correlations between plant and insect diversity, and contributions to ecological resilience – among others. But it’s much more difficult to quantify the specific functional differences between high-diversity and low-diversity prairie plantings. Even basic questions are difficult – for example, how many plant species does it take to see benefits?
Most of us who spend time in prairies know intuitively that plant diversity is important, but if we’re going to influence environmental policy, agricultural practices, and other large-scale conservation strategies, we’re going to need stronger and more quantified answers than intuition provides us.
In an attempt to help find some of those more specific answers, we have built some research plots within our Platte River Prairies, in which we’ve established prairie plantings of various plant diversity. Each treatment plot is 3/4 acre (1/3 ha) in size – big enough that we hope to compare patterns of invertebrate species composition and activity, soil changes, differences in the resistance to invasive species, and more. We’ve actually established two sets of plots now; one in 2006 and the second in 2010. The 2006 set consists of low diversity (15 species) and high diversity (100 species) plots, and the 2010 set consists of three treatments: a monoculture of big bluestem, low diversity (mostly grasses, with a few forbs), and high diversity (100 or more plant species). Each treatment is replicated at least 4 times.
Clint Meyer, of Simpson College (Iowa), was out last week doing some sampling of ground-dwelling invertebrates. Here he examines insects caught in a pitfall trap within our 2010 diversity research plots.
A close-up view of a vial of insects caught in a pitfall trap. There are least two species of ground beetles in the vial (floaters and sinkers!) as well as springtails (the little specks floating in the lower left hand portion of the vial).
To date, our research team has focused mainly on the 2006 plots, and has collected data on soils, nematodes, invertebrates, insect herbivory rates, rates of invasive species encroachment, and more. We’ve seen some intriguing patterns in terms of insect herbivory rates and invasive species encroachment, but have more work to do. This year we’ll start to collect our first data on the 2010 plots as well. So far, we’ve had researchers from the University of Nebraska-Lincoln, the Nebraska Cooperative Fish and Wildlife Research Unit, Simpson College, and Kansas State University. Several research papers have already been submitted to journals, and more are on the way. We’d love to add more researchers to the project, so please let me know if you’re interested in participating. We’re also looking for a PhD student or two to work on the project (see our earlier announcement).
Here are a few photos from last week, showing what the plant communities in the research plots look like:
This is a photo of one of our big bluestem monoculture plots, seeded in 2010. It consists mainly of big bluestem, with some Canada goldenrod and a few weedy wildflowers here and there.
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One of our 2010 low diversity plots. This treatment is dominated by several grass species, but has only a few scattered wildflowers.
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A high-diversity (2010) plot, showing big differences in both plant composition and habitat structure from the lower diversity treatments.
I have lots of research questions I’d like to explore with these plots, but I’m curious to hear what questions come to mind for you. Are there challenges you’re dealing with or issues you know of that we could help address by designing our research in a particular way? Or are there questions you’d ask just because you’re curious? I’d love to hear suggestions, and I’ll pass them on to the others working at the site.
The Prairie Ecologist 
It will be interesting to observe the trajectory of establishment of the arthropod diversity in the different treatments, i.e. community assembly. There’s a fair bit of work of this sort on pond mesocosms, much of it being done right around here by Washington University (St. Louis) ecologists, but not so much on terrestrial sites.
This is really neat!! I have a couple of questions that come to mind.
1) On establishment sites with previous annual nitrogen fertilization regimes, what is the impact on the soil organisms, and what is the impact on plant establishment in each plot??
2) What is the quality of cattle forage provided in each of the plots in relation to the diversity of plants and soil micro-organisms?? Is there a measureable difference in digestibility and nutrient content??
3) Where is the threshold of a cost/benefit value?? Meaning at what point do you get all the benefits of soil health, resilience, etc…while minimizing the cost passed on to a private landowner trying to purchase seed when purchasing local ecotype, or hand harvesting is out of the question?
Looking forward to your research.
Tim – great questions. I’ve struggled with the forage quality issue. We could do some simple clipping plots and have them analyzed, but that wouldn’t necessarily equate to the forage a cow might eat. It would, I guess equate to hay quality, which might be good enough… Anyway, I appreciate the ideas. I’ll see what we can do with them.
I’d be interested in knowing whether there is a relationship between plant diversity and subsurface soil moisture. I was also curious how this study was going to build on the work of Tillman’s group on the relationship between prairie diversity and resilience. I was really influenced by his Nature paper. The link for those who are interested is: http://cedarcreek.umn.edu/biblio/fulltext/t1189.pdf
Patrick – it’s a good question, and one I don’t know the answer to…yet. And we’re definitely working with Tilman’s projects as context. In fact, two of my collaborators (one on this particular project, and another on another related project) are former students of Tilman’s. Tilman used much smaller plots and a more controlled approach – for example, he kept his plots hand-weeded to keep the diversity at the same level over time. That’s a great approach for testing the pure ideas he wanted to test, but fairly unrealistic in a land management context. We’re testing some of the same ideas on a larger scale, with a more realistic model – it’s messy, because in a larger area, there’s more heterogeneity and room for variation. Will invasive species find a foothold in that varied community that they wouldn’t find in a small tightly controlled plot? Or is the plant diversity really the key, even at a large scale with lots of noise. That will be a key question. In addition, our larger size gives us the scale to ask some broader questionsa about habitat use, etc.
I’m curious if areas where cool season species have been introduced reduce the vigor of warm season species.
James – so far, we’ve not introduced those cool season grasses to a large enough area to see that kind of impact. It’s a very interesting question though. I’ll have to think about whether or not we can design something to investigate it.
What is the surrounding area around each treatment and does it effect the insect diversity simply by proximity?
It’s an excellent question. The plots are only separated from each other by about 21 feet of mowed trail. I’m sure many (most?) insect species can cross that with little problem. We’ll need to be (and have been) careful in the way we frame our questions and studies to take that into account.
I started my graduate career too late. I’m just about done, and you go and upgrade the richness experiments!
You do have manipulated species pools for the plots, so anyone interested in disturbance could evaluate the relative importance of random processes in community assembly after disturbances on the plots, or within treatments you have controlled species pools, so one could also study the relative importance of deterministic processes under different conditions. Doing both at the same time in the same place would be pretty excellent.
Not to make things more complicated but it would be interesting to note if there was a plant diversity level at which insect diversity levels off or if it continues to increase with an increase in plant diversity. This would likely mean you would have to add more plots with diversity levels at say 50 species, 75 species etc. If there was a level at which insects level off then it may help to determine mixes for landowners or even to help prairie managers restablish a funcitoning interaction of invertebrates to plant life.